1. Field of the Invention
The invention relates generally to a heat mode recording material and, more particularly, to a high resolution thermal imaging medium comprising a heat sensitive layer interacting, at an image-wise application of heat, with an image forming substance for producing images of very high resolution.
2. Description of the Prior Art
Unlike the image processing of conventional photographic materials using silver halide emulsions, thermal imaging media require neither a dark room nor any other protection from ambient light. Instead, images may be produced with thermal imaging media by the application of heat patterns corresponding to the image to be produced and, since these materials can provide images by quicker and simpler processes than those applicable to silver halide materials, they are more convenient and economical than conventional photographic imaging materials. Another consideration which contributes to their desirability is that unlike silver halide materials, thermal imaging media require substantially dry image developing processes and they are unaffected by sustained periods of elevated ambient temperatures. Moreover, thermal imaging media allow the making of more stable images of higher quality because they do not suffer from the image quality drift resulting from the wet processing and temperature effects of silver halide materials.
As thermal imaging media may be used with relative ease and in a potentially wide range of applications, proposals relating to their manufacture and use have not been lacking. One source of heat lately to have become conventional for exposing thermal imaging media are lasers of sufficient power output and appropriately modulated while scanning a medium in an image pattern. The time required for irradiating the medium in this manner is relatively short. Other materials use conventional heat sources such as, for instance, xenon flash tubes.
For instance, U.S. Pat. No. 4,123,309 discloses a composite strip material including an accepting tape comprising a layer of latent adhesive material in face-to-face contact with a layer of microgranules lightly adhered to a donor web. At least one of the layers bears a radiation absorbing pigment, such as carbon black or iron oxide, which when selectively heated in accordance with a pattern of radiation, momentarily softens adjacent portions of the adhesive material sufficiently for the latter completely to penetrate through the pigment. Upon separation of the accepting tape and donor web, microgranules are said to transfer to the accepting tape in the irradiated areas only.
A similar material is disclosed by U.S. Pat. No. 4,123,578.
U.S. Pat. No. 4,157,412 discloses a composite material for forming graphics which includes a layer of latent adhesive material, a mono-layer of granules lightly adhered to a donor web, and a thin layer of bonding material between and in face-to-face contact with layers of granules and adhesive. The layer of bonding material maintains the adhesive and granular layers in close proximity and excludes air from therebetween. When the composite material is selectively heated in graphic patterns, corresponding portions of the bonding layer melt and corresponding portions of the adhesive material and granular layer soften, absorb the melted portions of the bonding layer and adhere together. Upon subsequent separation of the layer of adhesive and the donor web the remaining portions of the layer of bonding material separate, whereas granules transfer to the accepting tape in the heated areas to provide the graphics.
In U.S. Pat. No. 4,547,456 a heat mode recording material is described which comprises a support and a heat sensitive layer positioned on the support, in which the heat sensitive layer comprises an ionomer resin obtained by ionically cross-linking with at least one metal ion, a copolymer comprising an alpha- olefin and an alpha methylene aliphatic monocarboxylic acid and a hydrophobias binder.
Other materials are known which instead of using a source of heat to provide an image which may be transferred from one layer to another by locally changing the adhesion of photohardenable image forming substances relative to the layers, rely upon actinic radiation for forming images. An example of such a material is disclosed in U.S. Pat. No. 4,247,619.
None of the known thermal imaging materials appear to have found wide acceptance, possibly because of the relatively complicated mechanism of the image-wise transfer of an image-forming substance from a donor layer to a receiving layer as a result of applied heat patterns. Other problems may be involved in the coherence of the image-forming substance which may not consistently yield images of a resolution sufficiently fine to be acceptable to consumers. Still further problems may result from the difficulty of removing microscopical irregularities and air gaps when using two separate donor and receiver webs. It appears that none of the thermal imaging materials currently available satisfy the demand for high photographic quality or high resolution required by industry.
It is, therefore, desirable to provide a thermal imaging medium of superior performance for forming images of high resolution by a simplified mechanism of image-formation.